Riser with a rigidity varying lower portion

ABSTRACT

A RISER, ARRANGED FOR COMMUNICATION BETWEEN A STRUCTURE AT THE OCEAN FLOOR AND EQUIPMENT ON THE SURFACE WHERE THE WATER DEPTH MAY BE, FOR EXAMPLE, MORE THAN 600 FEET, HAS A LOWER PORTION WITH A RIGIDITY OR STIFFNESS SUFFICIENT TO WITHSTAND THE WORST CONDITION BENDING MOMENT APPLIED THERETO. THE STIFFNESS OF THE UPPER PORTION IS DETERMINED BY THE DESIGN REQUIREMENTS FOR EFFICIENT AND ECONOMICAL PIPING AND COMMUNICATION BETWEEN THE OCEAN FLOOR AND THE OCEAN SURFACE. IN ADDITION, THE SECOND MODULUS OF THE LOWER PORTION DECREASES AT STATIONS DISPOSED FURTHER FROM THE OCEAN BOTTOM AND WHERE THE UPPER AND LOWER PORTIONS JOIN THE SECTION MODULUS ARE SUBSTANTIALLY THE SAME.

Sept. '20, 1971 3, w, R N 3,605,413

RISER WITH A RIGIDITY VARYING LOWER PORTION Filed Oct. 24, 1969 3Sheets-Sheet 1 INVENTOR. GEORGE W. MORGWN ATTORNEY Sept. 20, 1971 3, wMORGAN 3,605,413

RISER WITH A RIG'IDITY VARYING LOWER PORTION Filed Oct. 24, 1969 3Sheets-Sheet 2 INVIZN'IUR. GEORGE W. MORGAN ATTORNEY Sept. 20, 1971 9,w, MORGAN 3,605,413

RISER WITH A RIGIDITY VARYING LOWER PORTION 3 Shoots-Shoat ,1 3

FilBd Oct. 24, 1969 IN VENTOR. GEORGE W. MORGAN ATTORNEY United StatesPatent 3,605,413 RISER WITH A RIGIDITY VARYING LOWER PORTION George W.Morgan, Anaheim, Calif., assignor to North American Rockwell CorporationFiled Oct. 24, 1969, Ser. No. 869,313 Int. Cl. E21b 17/00 US. Cl. 61--4613 Claims ABSTRACT OF THE DISCLOSURE A riser, arranged for communicationbetween a structure at the ocean floor and equipment on the surfacewhere the water depth may be, for example, more than 600 feet, has alower portion with a rigidity or stiffness sufficient to withstand theworst condition bending moment applied thereto. The stiffness of theupper portion is determined by the design requirements for eflicient andeconomical piping and communication between the ocean floor and theocean surface. In addition, the section modulus of the lower portiondecreases at stations disposed further from the ocean bottom and wherethe upper and lower portions join the section modulus are substantiallythe same.

FIELD OF THE INVENTION This invention relates to a riser forcommunication between a point on the ocean floor and a point on thesurface, and more particular, to a riser that is subject to relativelylarge environmental lateral forces in any horizontal direction.

BACKGROUND In most long term operations under the surface of the waterand in many relatively short term operations, communication of varioustypes is required between the operating structures at or near the bottomand operating structures at the surface. Such communications may includetransfer of information, material, power, etc. which will be referred'toherein as utilities. Thus, in an underwater oil well, casing, cables,and other pipes, which will be referred to as a riser hereinafter, mustextend from anapparatus at the water surface to a structure at thebottom of the body of water.

Structural support for such a riser poses problems that are greatlymagnified with increasing water depth. The various pipes and cablesthemselves collectively or individually can be readily designed towithstand the forces acting through the ocean movement, since the riseris structurally designed to withstand the dynamic forces associated withactual sea state, hydrostatic forces associated with current and seadepth, and environmental forces which result in corrosion, abrasion anddegradation of structural material properties. Changes in the positionof a floating surface vessel to which the upper end of the riser isconnected will naturally move the upper end of the riser, and the pipesin the riser would be designed to also withstand this force. Theexternal forces, being unsteady and varying in magnitude and indirection along the length of the riser, cause the riser to deflectgreatly and an additional design problem is encountered since the bottomof the riser needs to withstand these excursions of the riser.

Attempts to solve or avoid these structural design problems haveresulted in consideration of a variety of different types of riserstructural concepts, such as a cantilevered structure such as a towerwhich is free standing and unencumbered by buoys and mooring devices.Structural size and weight in such an arrangement are inordinately greatin deep water. For example, for a depth of 2,000 feet the bottom of atower structure would have a 3,605,413 Patented Sept. 20, 1971 "icediameter on the order of twenty feet with a required wall thickness ofone and one-half inches. With such an arrangement overturning moment isresisted at the base so that major design problems exist, particularlyin ocean bottom with deep deposits of mud. Another concept that has beenconsidered is an articulated riser, articulated on one or more axes.This riser requires a hydrofoil shape and bearings to permit therequired rotation of the riser and to provide what is normally referredto as a pinned connection at the ocean bottom. Such bearings are notreliable over a long period of time, resulting in excessive drag andbending moments after the segments become locked. In another concept,the riser can be made fully flexible to permit a maximum amount ofcompliance with the load environment, but risers that are highlyflexible, such as rubber composites, are limited by current state of artto depths of about 800 feet.

Therefore an object of this invention is to provide a riser that is ofminimum size, maximum strength, and long life with little or nomaintenance.

Another object of this invention is to provide a riser having theadvantages of a flexible riser and also the advantages of a cantileveredriser cantilevered at the ocean floor.

Other objects and many of the attendant advantages of the invention willbecome more apparent from the following detailed description when takenin connection with the accompanying drawings wherein:

FIG. 1 is a schematic illustration not to scale of a system utilizing ariser constructed in accordance with principles of this invention;

FIG. 2 is an elevation partly insection, of a portion of the lowerportion of the riser of FIG. 1 compressed, for illustration purposes,more in the vertical direction than in the horizontal direction;

FIGS. 3 and 4 are cross-sections of the riser taken on lines 33 and 4-4respectively, in FIG. 2 as vie-wed in the direction of the arrows;

FIG. 5 is a cross-section of the riser taken on line 5-5 in FIG. 6, asviewed in the direction of the arrow;

FIG. 6 is a side view of the riser illustrating the structuralarrangement for transfer of tension from the upper portion of the lowerportion; and

FIG. 7 is a section in elevation illustrating aspects of the connectionof the bottom section of the riser to a bottom structure.

DESCRIPTION OF THE DRAWINGS As illustrated in FIG. 1 a riser constructedin accordance with the principles of this invention includes an upperportion 20 and a base or lower portion 10 fixedly secured to a suitablefixed structure 12 at an ocean bottom 13 and rising to the ocean surfaceillustrated at 14. The riser has an upper end thereof connected to amoored, floating surface facility 16 in a manner for example asdisclosed in a copending US. patent application No. 838,513 filed onJuly 2, 1969, and assigned to the same assignee as this application. Thebase portion 10 of the riser has a non-uniform rigidity or sectionmodulus that decreases from a maximum at its point of attachment to thestructure 12 to a minimum value at a tension transfer section 18, aswill be more particularly described below. At the tension transfersection 18 the flexural rigidity of the base portion has decreased to avalue substantially equal to that of the upper portion 20 which isrelatively flexible. The upper portion 20 is termed flexible since itslength to radius ratio is relatively large, for example, the length maybe on the order of 1500 feet or more and the radius would be on theorder of six inches. Accordingly, the structure will buckle under itsown weight and substantial tension is required to minimize the curvatureand minimize bending stresses in the steel pipes making the upperportion 20. A flexible riser constructed in accordance with theprinciples of this invention having a length of, for example, 1900 feet,would have the lower portion of a length of about 400* feet and formedof pipe with a wall thickness of 2 /2 inches and a diameter at its baseof 41 inches while the upper portion 20 may be 1500 feet long made of a12 inch diameter steel pipe having a Wall thickness of about 1 inchformed of high-test line pipe, API Standard, SLX, X60.

In this embodiment, substantially all of the tension is applied at theupper end of the riser, by means of a spherical float member 22 which issecured to the upper end of the flexible upper portion 20 but preferablyis located beneath the ocean surface 14 at a distance of about 100 feet,according to the site characteristics. Such a float will have a diametrof, for example, 30 feet. In some situations for the purposes of thisinvention the facility 16 may be moved and then the riser itself may betemporary terminated at the float 22 which accordingly would provide theentire tension load and, further, because the float is free floating, asubstantial amount of the required angular freedom or pinning of theupper end of the rises is inherently provided.

Because of the large size and weight of the lower portion 10 of theriser, supplementary flotation is required during the transportation andinstallation phases of riser, as is well known in the art. Floats 24 and26 are secured to the lower portion 10 to provide a predetermined amountof positive buoyancy when needed. However, the floats may be floodedafter installation to provide negative buoyancy and may be connected tothe surface facility so that the water may be blown out, if need be, toprovide buoyancy assistance in controlled rising in case of riserremoval.

The riser is schematically illustrated in FIG. 1 in an exaggeratedcurvature condition but is shown with the general configuration that itwill assume with normal excursion of the moored floating facility 16subject to wave motion and water currents which, as is known, have avelocity profile that decreases with depth. The floating facility 16 isillustrated in one position of extreme excursion, these excursions beinglimited by suitable moorings (not shown). The horizontal distance fromthe vertical line directly through the axis of the lower portion 10identifies the radius of excursion of the floating facility 16 whichexcursion may be in any horizontal direction depending upon watercurrent. Note that minimum radius of curvature occurs in the flexibleupper portion 20, whereas the radius of curvature gradually increases inthe lower portion 10 as the bottom attach point is approached. Byincluding a rigidity transition structure in the riser, in accordancewith the present invention, there is no abrupt change in curvature aswould be the case where a flexible member is connected to a more rigidmember. Thus, there is no discontinuity in curvature in the riser andparticularly at the transfer section 18. Furthermore the curvature ofthe base portion 10 varies somewhat, inversely with its flexuralrigidity or section modulus, from substantially zero curvature (infiniteradius of curvature) at the point of connection to the bottom structureto increasing curvatures (reduced radii of curvature at station furtherremoved from the base 12.

' portion 10 are changed to provide the desired section As illustratedin detail in FIG. 2 a steel pipe 30 having a diameter, as mentionedabove in the exemplary embodiment, of 12 inches and a wall thickness of1 inch, extends for the entire length of the riser, from the floatr22 toand through the lower portion 10. As will be readily understood to thoseskilled in the art, the flexural rigidity, defined as the product ofmodulus of elasticity and moment of inertia of a structure, may bevaried by varying the radius of a circular beam or the wall thicknessthereof or both. Since the lower portion is made of steel having amodulus of elasticity of 30,000,000 pounds per square inch, both radiusand wall thickness of the lower modulus or flexural rigidity transition.The portion 10 comprises a number of sections for example, threesections 32, 34, and 36 although a larger number of sections such as 10may beused. The sections include the base section 32, the intermediatesection 34 and the upper section 36. The base section 32 includes anannular enlarged flange 38 and a lower connection portion 40 arranged tobe fixedly secured to the fixed underwater structure 12, for example asshown in FIG. 7 the details of which form no part of this invention, butform part of another application which will be copending with thisapplication.

The ten sections in portion 10 of the to-be-fabricated embodiment havethe same inside diameters such as 36 inches, for example, with theuppermost section having an outside diameter of 36.75 inches and thelowermost section having an outside diameter of 41 inches. In such anarrangement intermediate sections have outside diameters 31.0, 37.25,37.75, 38.0, 38.50, 39.0, 39.5, and 40.0, respectively, to provide aprogressively increasing diameter and a progressively increasing pipewall thickness from the uppermost section to the lowermost section. Theseveral sections, for example sections 32, 35 and 36, are each connectedto the others in end to end relation by welding as indicated at 42. Thetop of each section is tapered such as at 43 to provide smoothtransition from one section to the adjacent section. This taper 43 canbe made to extend forexample from section 36 to section 32.

In order to provide for transfer of stress from the pipe 30, whichcarries the tension, to the upper section 36 and to provide rigiditytransition therebetween, a plurality of stress transfer members 44, 46,48 and 50 (FIG. 3) are provided. The stress transfer members compriseelongated flat plates disposed radially and each welded at its outeredge to the inner surface of the upper section 36 and'each welded at itsinner edge to the outer surface of the pipe 30. As most clearlyillustrated in connection with the stress transfer member 44 the membershave their inner edges longer than their outer edges. In the 1900 feetriser embodiment referred to above, the total length of each of thetransfer members 44, 46, 48 and 50 is fourteen feet, while the length ofthe outer edges welded to section 36 is less than 7 feet.

In order to facilitate and enhance the transfer of stresses, the upperend of the section 36 is provided with a plurality of upstanding toothsections such as tooth sections 52,54, and 56 more clearly illustratedin FIG. 6. As can be seen in this figure, each of the transfer members44, 46, 48 and 50 is connected to the outer pipe section 36substantially at a midpoint of a respective tooth section For furtherstiffness and improved stress transfer there are provided for each ofthe transfer members 44 through 50, a group of triangulated gussets 58,60, 62 and 63 (FIG. 3). The gussets have their base portions welded tothe upper edges of the respectivetooth sections extending to an apex andwelded to the outer sloping edges of the respective stress transferplates, 44, 48, 46 and 50.

As illustrated in FIGS. 3, 4, and 5 a plurality of additional pipes forexample, eight pipes, such as illustrated by pipes 61, 64, 65, 66, 70,72, 74 and 76, are carried by the riser to provide the necessarytransfer of utilities between the floating facility 16 and structure 12at the bottom of the body of water. A copending application of G. W.Morgan for Multi-Conduit Underwater Line, Ser. No. 721,014, filed Apr.11, 1968, now Pat. No. 3,526,- 086 describes the structural details ofsuch pipes to form a riser. The various conduits are secured by suitablemeans (not shown) to the pipe 30 at intervals spaced longitudinallyamong the pipe 30 and are preferably helically wound as strands arewound in a wire rope whereby the various pipes may move relative to eachother and to the pipe 30. This relatively loose attachment of the eightcarried pipes minimizes their bending stresses whereby they contributelittle, if at all, to the fiexural rigidity of the upper section of theriser. The eight pipes extend for the entire length of the riser alongthe main tension carrying pipe 30.

Referring to FIG. 7 a simplified attachment between the riser and thestructure 12 is schematically shown to give a more completeunderstanding of the usefulness and problems solved by this invention. Amore complete description of the novel features of this arrangement isdisclosed and claimed in a copending U.S. patent application titled,Undersea Riser Structure, by Walter Brown, and filed on Oct. 24, 1969,Ser. No. 869,078. On the bottom of the lower section 32 is formed thesleeve 40 which slips over a post 84 on the structure 12. A pair ofvertically spaced annular bearing projections 80 and 82 extendinginwardly thereof from sleeve 40 provide the required bearing surfacebetween the two members and allow for easy assembly. The center post 84has disposed therein a pipe 88 which is suitably coupled to pipe 30 bycoupling 90. Pipe 88 extends laterally through an opening 86 formed inone side of post 84. The section 32 of the cantilevered beam section maybe provided with a plurality of apertures of which one is indicated at92 to allow passage of the auxiliary pipes, for example pipe 61 to beconnected to pipe 4 by a coupling 96. Although only auxiliary pipe 61 isshown, the other auxiliary pipes, may be connected in a mannersubstantially identical to that illustrated for line 61.

There has been described an improved structural riser particularlyadapted for use in waters of great depth, on the order of 1,000 to 2,000feet or more, which achieves an optimum combination of stability,rigidity, durability, compactness and strength by providing a flexibleelement for a major portion of its length and a rigidity transitionelement from the flexible element to a fixed rigid connection at itsbottom portion.

What is claimed is:

1. A riser comprising:

an elongated structural member having a lower end adapted to be securedunder a body of water and an upper end adapted to be connected to afloating body;

said member having a first portion with relatively low rigidity andhaving a second portion with a controlled rigidity extending from saidlower end to the point of attachment with said first portion, saidcontrolled rigidity increasing from its value at the point of attachmentwith said first portion to a value at said first end to allow thedeveloped internal stresses to be below a given value and to allow saidsecond portion to assume a curvature that increases with distance fromsaid lower end.

2. An elongated structural tension member having:

an upper section extending over a major portion of its length, saidupper section having a stiffness that will not support its weight; and

a terminal section attached to said upper section and formed with astiffness that progressively increases from the point of attachment tothe end of said upper section. I

3. The member of claim 2 wherein:

said end of said terminal section is fixedly secured beneath a'body ofwater; and

the other end of said upper section is buoyantly carried above said endof said terminal section.

4. The member of claim 3 further including:

a main stress carrying element extending substantially the full lengthof the member; and

said terminal section including a secondary stress carrying elementproviding the progressively increasing stiffness.

5. The member of claim 4 wherein:

said secondary stress element is a tubular element extending around andradially spaced from said main element, said tubular element having itsupper end 6 formed with a plurality of longitudinally extending teeth;and

a plurality of stress transfer elements, each comprising a plateradially connected between said main element and the upper end of saidtubular element.

6. The member of claim 5 wherein:

said main stress carrying element comprises a main pipe encircled by aplurality of pipes;

said tubular element enclosing said plurality of pipes and comprising aplurality of tubular sections rigidly connected in end to end relation,all tubular sections having the same internal diameter and each sectionhaving a smaller external diameter than the next lower section.

7. The member of claim 4 further including:

a plurality of pipes carried by said main stress carrying element;

said secondary stress carrying element comprising a tube encircling saidmain stress carrying element to provide a space therebetween; and

means for transferring stress between said main stress carrying elementand the upper end of said tube.

8. The member of claim 7 wherein:

said tube comprises a plurality of sections rigidly connected in end toend relation;

all of said sections having the same internal diameter; and

each of said sections having a smaller external diameter than the nextlower section.

9. An underwater riser that may be fixedly connected at one endunderwater and at the other end to a floating body, whereby the riser issubject to forces and displacements due to wave motion and watercurrents, comprlsmg:

first means at said one end that can be fixedly secured to an underwaterstructure;

second means at said other end that can be connected to said floatingbody for exerting tension upon the riser;

third means extending from said other end for a major portion of saidriser and said third means having a relatively low rigidity so that itcannot support its own weight; and

fourth means disposed between said first means and third means andhaving a varying rigidity that varies from a minimum value at the pointof attachment to said third means to a maximum value at said firstmeans.

10. A riser comprising:

a flexible upper portion of relatively low rigidity;

a bottom portion of relatively great rigidity; and

an intermediate transition portion between the upper and bottom portionshaving a rigidity that varies from the value of said bottom portion tothe value of said upper portion.

11. A riser comprising:

a tension member;

means for exerting tension on the member at one end thereof; and

means for securing the other end of the member to a fixed underwaterstructure, said securing means comprising a cantilevered beam fixed atone of its ends to said underwater structure and attached at the otherend to the tension member, said beam having a rigidity that increasesfrom a lower value at said other beam end to a substantially highervalue at the end thereof connected to the underwater structure.

12. A tensioned riser comprising:

a lower portion fixed to a structure at the bottom of a body of waterand constructed to act as a cantilevered beam of decreasing rigidity;and

an upper portion, continuous with the lower portion,

and constructed to act as a tensioned cable.

13. A riser comprising: a first tubular member extending the entirelength of the riser; second tubular member concentric with the firstmember and encircling the first member for a substantial portion of thelower length of the first member, said second tubular member comprisinga plurality of sections rigidly connected to each other in end to endrelation wherein each of the sections has substantially the same insidediameter and each has a wall thickness that is substantially less thanthe wall thickness of the next lower section of the second tubularmember;

the uppermost section of the second tubular member having the upper endthereof formed with a plurality of axially extending teeth;

a plurality of stress transfer members each comprising an elongatedplate disposed radially and connected to the respective teeth of saiduppermost section References Cited UNITED STATES PATENTS Rhodes et a1.175- 7 Triplett 166-.5 Howard 166-.5 Joubert et a1. 166--.5 Morgan166--.5X

J. KARL BELL, Primary Examiner US. Cl. X.R.

